The present disclosure relates to a length artifact to determine the measurement errors associated with a three-dimensional (3D) measurement instrument and with methods of measurement using the length artifact.
Portable metrology instruments, such as portable laser trackers and articulated arm coordinate measuring machines (AACMMs), find widespread use in the manufacturing or production of parts where there is a need to rapidly and accurately verify the dimensions of the parts during various stages of the manufacturing or production (e.g., machining). Portable metrology instruments represent an improvement over known stationary or fixed, cost-intensive and relatively difficult to use measurement installations, particularly in the amount of time it takes to perform dimensional measurements of relatively complex parts.
A laser tracker is in a class of instruments that measure coordinates of a point by sending a beam of light to the point. The beam of light may impinge on a retroreflector target in contact with the point. The laser tracker determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter (ADM) or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. A related instrument is a total station (tachymeter) that measures to either a retroreflector or a point on a diffusely scattering surface. Laser trackers, which typically have accuracies on the order of a thousandth of an inch and as good as one or two micrometers under certain circumstances, are usually much more accurate than total stations. The broad definition of laser tracker, which includes total stations, is used through this application.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface on which the SMR rests remains constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
Periodically it is desirable to evaluate the performance of a 3D measuring instrument. One way to do this is to measure a length artifact, also known as a length standard or a reference length, and to compare a length of the artifact as obtained from a calibration to the length as obtained from a measurement with the 3D measuring instrument. The difference between the measured and calibrated length values is the error, which is an indicator of performance of the 3D measuring instrument. Today, there is a need for a length artifact that is stable, easily calibrated, and easily measured with a 3D measuring instrument such as a laser tracker, articulated arm CMM, or similar device.
Accordingly, while existing length artifacts are suitable for their intended purposes, the need for improvement remains, particularly in providing an improved length artifact and method of measuring with a 3D measuring device.